The present invention relates to a method for controlling a retarder brake of a motor vehicle.
Retarder brakes are customarily used as engine brakes and/or primarily in utility vehicles. As a general rule, the retarder brake acts only on one axle, namely the driven one. Principally in specific vehicles for which retarder brakes with high braking effects are required, a need exists for a high degree of adhesion at the wheels of the constantly braked axle. When the road conditions are bad or the vehicle is only partially loaded, a tendency to lock can therefore occur at the constantly braked axle.
For vehicles which have an anti-lock brake system (ABS) with associated wheel speed sensor system, when there is a commonly activated service brake and retarder brake it is known to switch off the retarder brake when there is a threat of a wheel locking and with the activation of the ABS. Such a brake system is described in Published Examined Application DT 23 22 446 B2. The retarder brake is reactivated after such switching off when a braking pressure measured at a specific point in the brake fluid circuit drops below a prescribed threshold value.
In DE 29 31 491 C2 a retarder brake is described in which the anti-lock actuation signals of an ABS which indicate that a wheel is tending to lock are used to reduce the effect of an activated retarder brake even when the service brake is inactive. A control memory is activated instead of the anti-lock actuation signals and has the function in the ABS control unit of activating specific logical criteria. When actuation signals are first received in the ABS control unit, the control memory is set and then switches a control unit of the retarder brake for the purpose of allowing different characteristics of the service brake and retarder brake.
In a brake system described in DE 41 24 496 A1 having ABS and traction control (TCS), an electro-regenerative system which acts as a retarder brake is switched off for the duration of ABS control phases.
In DE 42 25 080 A1, a brake system contains a combination of a hydraulic and an electro-regenerative brake system which can be commonly actuated by an ABS configured to prevent locking of the rear axle in the case of panic braking and to control the braking effect of the rear axle in the case of normal braking, specifically as a function of the desired deceleration, the load and the present grip. In principle, it is predominantly the electro-regenerative brake circuit which is used for recovering energy in this brake system, and the hydraulic brake circuit is only activated in specific travel situations as an additional contribution to braking.
In DD 266 771 A5, a method for non-locking actuation of a hydrodynamic deceleration brake is provided in addition to the service brake. The angular speed of the driven wheels is continuously identified and evaluated. If excessive slip occurs at the driven wheels, the braking torque of the hydrodynamic deceleration brake is reduced until the risk of locking has receded.
In DE 38 29 951 A1, a method for load-dependent braking pressure control automatically distributes braking pressure to the axles of the vehicle as a function of inter-axle wheel speed signals in a wheel slip range below a point where a wheel starts to tend to lock and thus before an ABS control is activated. In this method, the distribution of braking pressure is left unchanged in each case as long as the absolute value of the difference between the inter-axle wheel speed values has not exceeded a prescribed limit value while, after such a limit value has been exceeded, the distribution of braking pressure is adjusted in a manner which reduces this excess. Thus, control of the distribution of braking force is achieved even far below the locking limit, i.e. in a range in which a customary ABS is not yet effective, e.g. in the range of inter-axle deviations in wheel speed between 1% and 7%.
In DE 40 10 551 A1, a method for improving the braking behavior of a motor vehicle prevents a locking state at a driving wheel which rolls with a low coefficient of adhesion with the activation of a braking device which is configured as a retarder and is connected upstream, as viewed in the drive direction, of a differential gear unit of a driven wheel axle.
For this purpose, the difference between the wheel speeds of the left-hand and right-hand wheel of this vehicle axle which is influenced by the braking device is identified and compared with a prescribed threshold value. If the absolute value of the difference in wheel speeds lies above the threshold value, the compensation movement of the differential is reduced or blocked. Measures against the continuously braked axle being overbraked on both sides are not provided here.
An object of the present invention is to provide a vehicle retarder brake control method by means of which locking states of the vehicle wheels caused by the effect of the retarder brake are reliably avoided.
This problem has been solved in accordance with the present invention by means of a method in which a braking effect of the retarder brake as a function of an absolute value of a difference between a wheel speed value derived from wheel speeds of at least one wheel influenceable by the retarder brake and a wheel speed value derived from wheel speeds of at least one wheel independent from influence of the retarder brake, and reducing the braking effect when the absolute value of the difference between the wheel speeds exceeds a prescribed limit value which is lower than the absolute value of the difference between the wheel speeds when one of the wheels has a tendency toward locking.
In this new method, the braking force of the activated retarder brake is already reduced before one of the wheels which can be acted on by the retarder brake starts to tend to lock in that this reduction already takes place when the absolute value of the difference between inter-axle wheel speed values exceeds a prescribed limit value which is smaller than the absolute value of the difference between the inter-axle wheel speeds when one of the wheels starts to tend to lock. As a result of this earliest practical reduction in the braking effect of the retarder brake, there is, in contrast with conventional systems in which the retarder brake is only switched off with an ABS signal which indicates the tendency to lock, no risk of excessively late switching off of the retarder brake owing to the comparatively slow reduction in braking torque of the retarder brake.
An advantage of the present invention is that it prevents locking of wheels owing to the braking effect of the retarder brake. Swerving of the continuously braked axle, with the associated risk of skidding, is reliably counteracted by this novel method both with single vehicles and also with lorries and semitrailers.
By virtue of the information on the difference between wheel speeds being used, the present invention makes it unnecessary to sense braking forces or absolute values of the coefficient of adhesion or of the wheel slip.
By specifying a speed-dependent limit value for the absolute value of the difference between wheel speeds, a higher degree of utilization of the adhesion for the effect of the retarder brake is made possible at lower vehicle speeds. Analogously, optimum adaptation of the starting time for the reduction of the effect of the activated retarder brake as a function of the desired braking for the service brake is now achievable. A monotonously decreasing dependence is understood here to be a profile which is constant or decreases in certain areas.
As a result of the dependence of the degree of reduction in the constant braking effect on the gradient of the absolute value of the difference between wheel speeds which increases over time, the reduction can be favorably adapted to the respective profile of a braking process and the retarder brake can be reset comparatively quickly to a suitable value.
According to another aspect of the present invention, a cyclical execution of the method may be advantageously provided in which the absolute value of the difference between inter-axle wheel speeds is monitored in each cycle and the braking effect of the retarder brake is decreased incrementally in each cycle as long as the absolute value of the difference between the inter-axle wheel speeds is greater than the respective limit value.
In a further development of the present invention, after a preceding reduction in the constant braking effect during a braking process, the constant braking effect is raised again if the absolute value of the difference between the inter-axle wheel speeds drops below the limit value by a prescribable value. As a result, a constant braking component which is the maximum possible for each loading state and road state is effectively maintained.